The radioactive properties of 14C are well known and have given rise to technologies that include the technology of radiocarbon dating. This technology is widely practiced on a laboratory scale, particularly to determine the age of specimens of interest. The results are typically expressed as “percent modem carbon” or pMC, the percentage of the measured activity in the sample to that in a standard, based on the activity in wood grown in the year 1850, i.e. at the onset of the industrial revolution. The pMC value is then interpreted in terms of the age of the sample, generally measurable by the use of this technology to a period some tens of thousands of years from the time of measurement, usually expressed as a time before the present (BP).
Interest in radiocarbon techniques goes considerably beyond academic curiosity and many practical applications now exist. Among them is their use in the discipline of isotope hydrology. Radiocarbon, produced in the atmosphere, enters the biosphere through photosynthesis. Carbon dioxide, generated by biological activity in the soil, dissolves in infiltrating rain water to become part of the dissolved inorganic carbon in ground water. The decay of radiocarbon in this partially closed system gives a measure of the residence time, or “age” of ground water. This is unique information for hydrologists studying the dynamics of ground water movement.
Initially, when the radiocarbon element 14C was discovered in the environment in the middle of the previous century, its presence was quantified by gas proportional counting. In this technique, a gas is produced from the carbon-containing sample which is introduced into a metal cylinder with a very fine wire along its axis, insulated from the cylinder, to which a high voltage is applied. Electric charges produced by radiation, such as from the decay of 14C, are attracted to the wire, producing impulses which can be recorded, or “counted.” Some time later the technique of liquid scintillation counting was developed. This involves the use of sensitive photomultipliers to detect and measure small light emissions that follow the excitement of a liquid scintillator by the energy deposited by ionizing radiation, such as occurs in samples containing radiocarbon when atoms of 14C decay by beta-radiation (that is, nuclear electron radiation) to nitrogen-14.
The liquid scintillation counting technique has been developed to a high degree of sophistication using modem computer technology. State-of-the-art, automated scintillation spectrometers are now marketed, vying in their sensitivity and accuracy with the classic, but cumbersome and labor-intensive, technique of gas counting.
The preparation of samples for use with liquid scintillation equipment is relatively time-consuming and tedious, involving numerous discrete steps, some of which call for manual intervention at various points. Although some of this work is repetitive and does not call for particularly skilled personnel to be involved, it is nevertheless precise and meticulous work.
Liquid scintillation counting relies on the use of a “cocktail” of organic substances that can undergo molecular excitation by absorbing energy deposited in the material by ionization caused by nuclear radiation, transferring this energy to a material which releases it as photons or quanta of light. Until the late 1980's carbon samples with environmental levels of radiocarbon were introduced into such a cocktail through the synthesis of benzene. This is an involved and relatively expensive process.
However, in the late 1980's an absorption technique, previously used for small, high-activity tracer samples, was adapted by Aravena et al. to environmental samples and is described in their paper entitled, “New Possiblities for 14C Measurements by Liquid Scintillation Counting,” in Radiocarbon, Vol 31, No. 3, 1989, pp. 387-393. This method was described in greater detail by Qureshi et al. in a paper, “The CO2 absorption method as an alternative to benzene synthesis method for 14C dating,” in Applied Geochemistry, Vol 4, pp. 625-633, 1989. The latter reference describes how the CO2 produced by appropriate processes from the carbon-containing samples and suitably purified, may be bubbled from a supply vessel through a mixture of a scintillation cocktail and an alkaline absorbant which binds the CO2 in the form of a soluble carbamate. Using this method, in order to load an adequate amount of CO2 into the cocktail, the bubbling process is continued until the cocktail is saturated, i.e. until the rate of absorption drops to zero. The mass of CO2 actually absorbed is determined by measuring the weight of the cocktail before and after the absorption process.
The method generally described in the aforementioned references has the advantage of much greater simplicity and is less time-consuming than the earlier benzene method. Although a considerably smaller amount of CO2 is accommodated in the counting cocktail, implying less sensitivity, it is still useful for a variety of applications in which high precision is not required, such as hydrology. Nevertheless, shortcomings have been noted. Not all the gaseous CO2 bubbling through the absorbent cocktail is immediately bound with the absorbent, and some of the CO2 is allowed to flow to waste. As a result, substantially more CO2, approximately 20%, has to be produced than can be accommodated in the liquid absorbent. Furthermore, as the pressure in the CO2 supply vessel drops, nitrogen must be added as carrier gas to maintain the pressure. Conditions have to be kept constant in order to ensure the same saturation conditions between different preparations. Even so, the degree of saturation ultimately achieved has to be established by carefully measuring the sample weight before and after each run, because the amount of CO2 absorbed under the same conditions can vary from run to run by as much as 3%. Moreover, when a specimen sample produces less than the standard amount of CO2 required for this process, “dead” CO2 (that is, 14C-free CO2) has to be added to the sample to make it up to a standard volume.
Further improvements to the above described methods were described by Leaney et al. in their paper, “New developments for the direct CO2 absorbtion method for radiocarbon analysis” in Quaternary Geotechnology/Quaternary Science Review, vol. 13, 1994, pp. 171-178. Their main contribution to the method was to add a plastic bladder to hold the gas sample, and a circulation pump to repeatedly pass the gas through the cocktail by bubbling until saturation is achieved. The method is more efficient, requiring a smaller CO2 excess and it more accurately ensures saturation, and hence the reproducibility of the amount of CO2 absorbed. This may obviate the need to measure the amount of CO2 absorbed in every sample, which is, at best, an imprecise procedure.
However, various factors complicate the method of Leaney et al. As the gaseous CO2 is repeatedly passed through the absorbent cocktail, a significant amount of vapor from the cocktail is entrained in the gas, and must be collected in a moisture trap. This requires the cocktail from the bubbler to be transferred to the moisture trap after absorption, whence the combined amount is transferred into a scintillation counting vial. Furthermore, in Leaney's method, a carrier gas must be added when pressure in the supply vessel drops, and dead CO2 must be added to samples smaller than the standard amount. Furthermore, the apparatus must be thoroughly cleaned, for example with a solvent before the next sample is produced.
Variants of the methods described above are presently in use in various laboratories worldwide, so that a typical current radiocarbon analysis would entail the following.
First, a sample is extracted from the specimen. One or more representative samples are extracted from the specimen for examination. The specimen could entail bone, charcoal, wood etc. for archaeological dating; carbonate, peat etc. for environmental studies; or a carbonate precipitate extracted from water for hydrological investigations.
Second, gaseous CO2 is generated from the sample. The purpose of this step is to free the carbon from the sample and extract it in gaseous form for use in later steps. For this step, therefore, the sample, if not already in a gaseous form, is converted by a suitable process into carbon dioxide (CO2) gas. Organic carbon-containing materials are combusted; inorganic carbon-containing samples (carbonates) are treated with acid. The resulting CO2 is then purified of nitrogenous and sulphurous impurities, of air, and dried in an appropriate gas transfer line.
Third, the carbon dioxide is converted into a form suitable for 14C counting. The CO2 gas may be synthesized into benzene through a series of chemical steps. The benzene is mixed with an organic liquid scintillation cocktail suitable for counting in a liquid scintillation spectrometer. Alternatively, in the method described by Qureshi and Aravena (and modified by Leaney), the CO2 may be absorbed into an alkaline absorbent cocktail for counting by scintillation spectrometer. The components of this cocktail are commercially available. The absorbent may be Carbosorb®, and the scintillation cocktail may be Permafluor E®—both by Packard Instrument Co., Meriden, Conn. A typical low-level liquid scintillation spectrometer is the Packard TriCarb® 2770, also by Packard Intrument Co. The vial containing the absorbed sample/scintillation cocktail mixture is placed in the spectrometer, and the scintillations caused by the 14C decays are recorded.
However, there are drawbacks to the current standard technology that can be summarized as follows. 1) Because the amount of CO2 gas that will actually be absorbed cannot be assessed beforehand, the method must rely either on measuring the mass of CO2 in the cocktail after it has been absorbed, or on replicating conditions (such as temperature) exactly, so that the same amount of CO2 is absorbed every time. 2) Saturating the cocktail requires either that some of the gas sample must flow to waste, or that the sample must be repeatedly re-circulated. 3) To ensure adequate transport of the CO2 from a supply vessel at low pressure, an inert carrier gas such as nitrogen must be added. 4) The step of bubbling gaseous CO2 through the cocktail is cumbersome. Vapour from the cocktail is entrained in the gas stream, adding to inaccuracies in assessing sample amounts. 5) The cocktail with absorbed CO2 (carbamate) must be transferred from the preparation line to a counting vial. Because the liquid has become rather viscous, the entire amount cannot be readily transferred, leading to inaccuracy. 6) The preparation line must be thoroughly cleaned after each sample in order to avoid cross-contamination (memory effect). 7) When the amount of CO2 derived from the test sample is less than the amount required to saturate the cocktail, “dead” CO2 must be added in known proportion. This leads to inaccuracies because it depends on two pressure measurements: that of the sample and that of the mixture.
Thus, a need exists for an improved apparatus and method for preparing samples for radiocarbon dating that will simplify the pre-existing methodology, increase the efficiency of transferring sample gas into a useful counting cocktail, and increase the accuracy of assessing the amount of sample gas transferred. It is believed that the present invention addresses these and other needs.